Why do LibraryFormats declare es instead of esm, where as ModuleFormat define both?

[christopher-francisco-jpmc]

In order to have {entryName}.esm.js extension, I have to manually do string manipulation

        formats: ['es', 'cjs'],
        fileName(format, entryName) {
          const finalFormat = format === 'es' ? 'esm' : format;

          return `${entryName}.${finalFormat}.js`
        },

Obviously I'm missing something very basic here. Is .esm.js not the standard extension? (see rollup wiki)

Moreover, the type definition for both formats (LibaryrFormats) and format (ModuleFormat) in the snipped above are as follow:

type LibraryFormats = 'es' | 'cjs' | 'umd' | 'iife' | 'system';

export type InternalModuleFormat = 'amd' | 'cjs' | 'es' | 'iife' | 'system' | 'umd';
export type ModuleFormat = InternalModuleFormat | 'commonjs' | 'esm' | 'module' | 'systemjs';

So both es and esm can potentially be values of format; but only es is a valid value to be passed to formats.

What am I missing?

[romhenri]

The reason "es" is used in LibraryFormats is because it is the standard abbreviation for ECMAScript modules in Rollup. "esm" is not a format recognized by default, but is often used as a file extension. In ModuleFormat, "es" and "esm" are included because ModuleFormat supports more custom options, while LibraryFormats follows standard format identifiers like "es" and "cjs".